Guava EventBus 使用介绍及自实现详解

使用 guava 工具,先在 pom 中引入依赖:https://mvnrepository.com/artifact/com.google.guava/guava/

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<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>29.0-jre</version>
</dependency>

Guava EventBus 的使用

简单使用

先编写监听者:

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import com.google.common.eventbus.Subscribe;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:08
* @description:
*/
public class GoogleEventBusListener {

@Subscribe
public void method1(String event) {
System.out.println("GoogleEventBusListener.method1, event = " + event);
}

@Subscribe
public void method2(Integer event) {
System.out.println("GoogleEventBusListener.method2, event = " + event);
}
}

再创建 EventBus 对象,将上述监听者注册到 EventBus 对象实例中,调用 post() 发送消息即可:

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import com.google.common.eventbus.EventBus;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:08
* @description: EventBus 简单示例
*/
public class SimpleEventBusExample {

public static void main(String[] args) {
EventBus eventBus = new EventBus();

// 监听器注册到 eventBus
eventBus.register(new GoogleEventBusListener());

// 发送消息
eventBus.post("this message from eventBus");
eventBus.post(200);
}

}

EventBus 构造函数中,如果不显示指定名称,则使用:default 作为默认名称。

注意:消息类型必须是包装类型,不能是基本数据类型。

多个监听者存在继承关系

当只注册监听器中的父类中也监听了某些消息的时候,父类的监听方法也会执行。如果父类的监听方法被子类覆盖监听,则只执行子类监听器的方法。

类监听器:

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import com.google.common.eventbus.Subscribe;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:18
* @description: 父类监听器
*/
public class FatherListener {

@Subscribe
public void accept(String event) {
System.out.println("FatherListener.accept, event = " + event);
}

@Subscribe
public void consume(String event) {
System.out.println("FatherListener.consume, event = " + event);
}
}

子类监听器:

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import com.google.common.eventbus.Subscribe;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:19
* @description: 子类监听器
*/
public class ChildListener extends FatherListener {

@Subscribe
@Override
public void accept(String event) {
System.out.println("ChildListener.accept, event = " + event);
}

@Subscribe
public void consumeTask(String event) {
System.out.println("ChildListener.consumeTask, event = " + event);
}
}

注册监听器并发送消息:

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import com.google.common.eventbus.EventBus;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:21
* @description: 多个监听器监听消息
* 当只注册监听器中的父类中也监听了某些消息的时候,父类的监听方法也会执行。
* 如果父类的监听方法被子类覆盖监听,则只执行子类监听器的方法
*
*/
public class MultipleListenerExample {

public static void main(String[] args) {
EventBus eventBus = new EventBus();
// 监听器注册到 eventBus
eventBus.register(new ChildListener());

// 发送消息
eventBus.post("this message from eventBus");
}
}

日志输出:

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ChildListener.accept, event = this message from eventBus
ChildListener.consumeTask, event = this message from eventBus
FatherListener.consume, event = this message from eventBus

多个事件存在继承关系

event 如果有父类,此时某个监听器监听的event是这个基类,那么消息是这个子类的时候,该监听父类的监听器方法也会执行。

父类事件对象:

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import com.google.gson.Gson;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:33
* @description: 父类事件对象
*/
public class FruitEvent {

String name;

public FruitEvent() {

}

public FruitEvent(String name) {
this.name = name;
}

@Override
public String toString() {
return new Gson().toJson(this);
}
}

子类事件对象:

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import com.google.gson.Gson;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:35
* @description: 子类事件对象
*/
public class AppleEvent extends FruitEvent {

public AppleEvent() {
}

public AppleEvent(String name) {
super(name);
}

@Override
public String toString() {
return new Gson().toJson(this);
}

}

监听器对象:

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import com.google.common.eventbus.Subscribe;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:32
* @description: 一个监听子类对象,一个监听父类对象
*/
public class SimpleListener {

@Subscribe
public void acceptFruitEvent(FruitEvent fruitEvent) {
System.out.println("SimpleListener.acceptFruitEvent => fruitEvent " + fruitEvent);
}

@Subscribe
public void acceptAppleEvent(AppleEvent appleEvent) {
System.out.println("SimpleListener.acceptAppleEvent => appleEvent " + appleEvent);
}
}

单元测试:

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import com.google.common.eventbus.EventBus;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:32
* @description: event 如果有父类,此时某个监听器监听的event是这个基类,
* 那么消息是这个子类的时候,该监听父类的监听器方法也会执行
*/
public class MultipleEventExample {

public static void main(String[] args) {
EventBus eventBus = new EventBus();

eventBus.register(new SimpleListener());

eventBus.post(new AppleEvent("apple"));

eventBus.post(new FruitEvent("fruit"));
}
}

日志输出:

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SimpleListener.acceptAppleEvent => appleEvent {"name":"apple"}
SimpleListener.acceptFruitEvent => fruitEvent {"name":"apple"}
SimpleListener.acceptFruitEvent => fruitEvent {"name":"fruit"}

异常处理

多个监听器同时监听相同消息,其中一个监听器有异常,那么不会影响其他监听器消费消息。可以在EventBus构造函数中传入一个异常处理器,当监听器有异常抛出时,会执行这个异常处理器。

思考点:如果异常处理很长时间,会不会导致后面的监听器等待执行?

异常处理器:

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import com.google.common.base.Throwables;
import com.google.common.eventbus.SubscriberExceptionContext;
import com.google.common.eventbus.SubscriberExceptionHandler;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:54
* @description: 异常处理器
*/
public class MySubscriberExceptionHandler implements SubscriberExceptionHandler {

@Override
public void handleException(Throwable exception, SubscriberExceptionContext context) {
System.out.println("======== handleException start =========");
System.out.println("exception => " + Throwables.getStackTraceAsString(exception));
System.out.println("EventBus => " + context.getEventBus());
System.out.println("Subscriber => " + context.getSubscriber().getClass().getName());
System.out.println("SubscriberMethod => " + context.getSubscriberMethod());
System.out.println("======== handleException end =========");
}
}

监听器对象:

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import com.google.common.eventbus.Subscribe;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:49
* @description:
*/
public class MyListener {

@Subscribe
public void method1(Integer sum) {
System.out.println("MyListener.method1 => " + sum);
}

@Subscribe
public void method2(Integer sum) {
System.out.println("MyListener.method2 => " + sum);
}

@Subscribe
public void method3(Integer sum) {
int i = sum / 0;
System.out.println("MyListener.method3 => " + sum);
}

}

单元测试:

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import com.google.common.eventbus.EventBus;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 22:48
* @description: 多个监听器同时监听相同消息,其中一个监听器有异常,那么不会影响其他监听器消费消息
* 可以在EventBus构造函数中传入一个异常处理器,当监听器有异常抛出时,会执行这个异常处理器
*
* 思考点:如果异常处理很长时间,会不会导致后面的监听器等待执行
*
*/
public class ExceptionListenerExample {

public static void main(String[] args) {

EventBus eventBus = new EventBus(new MySubscriberExceptionHandler());

eventBus.register(new MyListener());

eventBus.post(Integer.MAX_VALUE);
}
}

日志输出:

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MyListener.method2 => 2147483647
MyListener.method1 => 2147483647
======== handleException start =========
exception => java.lang.ArithmeticException: / by zero
…… // 打印日志栈信息

EventBus => EventBus{default}
Subscriber => org.woodwhales.guava.eventbus.demo4.MyListener
SubscriberMethod => public void org.woodwhales.guava.eventbus.demo4.MyListener.method3(java.lang.Integer)
======== handleException end =========

DeadEvent 实现类型

监听器监听 DeadEvent 类型事件:

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import com.google.common.eventbus.DeadEvent;
import com.google.common.eventbus.Subscribe;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 23:05
* @description:
*/
public class DeadEventListener {

@Subscribe
public void handle(DeadEvent deadEvent) {
System.out.println("DeadEventListener.handle deadEvent => " + deadEvent);
System.out.println("DeadEventListener.handle event => " + deadEvent.getEvent());
System.out.println("DeadEventListener.handle source => " + deadEvent.getSource());
}
}

单元测试:

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import com.google.common.eventbus.EventBus;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.2 23:07
* @description: 监听DeadEvent消息类型,那么可以从这个DeadEvent对象中获取事件源和eventBus
*/
public class DeadEventExample {

public static void main(String[] args) {
EventBus eventBus = new EventBus("DEAD-EVENT-BUS");

eventBus.register(new DeadEventListener());

eventBus.post("message");
}
}

日志输出:

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DeadEventListener.handle deadEvent => DeadEvent{source=EventBus{DEAD-EVENT-BUS}, event=message}
DeadEventListener.handle event => message
DeadEventListener.handle source => EventBus{DEAD-EVENT-BUS}

自实现 EventBus

Bus 接口设计

自己实现一个 EventBus 前需要先设计好需要多少功能,即多核心少接口:将监听者注册bus,将监听者从bus中移除,发送消息,发送带有主题的消息,获取当前 bus 名称等:

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/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.4 13:55
* @description: Bus 接口
*/
public interface MyBus {

/**
* 注册到 bus
* @param subscriber 订阅者
*/
void register(Object subscriber);

/**
* 从bus中取消注册
* @param subscriber
*/
void unRegister(Object subscriber);

/**
* 发送消息
* @param event
*/
void post(Object event);

/**
* 发送消息
* @param event
* @param topic
*/
void post(Object event, String topic);


/**
* 关闭bus
*/
void close();

/**
* 获取当前bus名称
* @return
*/
String getBusName();

}

Bus 接口实现

实现 Bus 接口,重要的是怎么来保存注册到中的监听者,怎么在消息发布者调用 post() 发布消息的时候,根据消息的类型找到对应的要监听的方法并执行,如果有异常处理器,则执行异常处理器。

MyEventBus 实现 MyBus 接口,在其类中增加以下属性:MyEventBus 的构造函数仅仅是设置属性的入口,消息存储和消息消费,均由 MyRegistry 和 MyDispatcher 来执行。

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public class MyEventBus implements MyBus {

private final MyRegistry registry = new MyRegistry();

private String busName;

private static final String DEFAULT_BUS_NAME = "DEFAULT";

private static final String DEFAULT_TOPIC = "default-topic";

private final MyDispatcher dispatcher;

public MyEventBus() {
this(DEFAULT_BUS_NAME, null, MyDispatcher.SEQ_EXECUTOR_SERVICE);
}

public MyEventBus(MyEventExceptionHandler eventExceptionHandler) {
this(DEFAULT_BUS_NAME, eventExceptionHandler, MyDispatcher.SEQ_EXECUTOR_SERVICE);
}

public MyEventBus(String busName, MyEventExceptionHandler eventExceptionHandler, Executor executor) {
if(Objects.isNull(busName) || busName.isEmpty()) {
busName = DEFAULT_BUS_NAME;
}
this.busName = busName;
this.dispatcher = MyDispatcher.newDispatcher(executor, eventExceptionHandler);
}

@Override
public void register(Object subscriber) {
this.registry.bind(subscriber);
}

@Override
public void unRegister(Object subscriber) {
this.unRegister(subscriber);
}

@Override
public void post(Object event) {
this.post(event, DEFAULT_TOPIC);
}

@Override
public void post(Object event, String topic) {
this.dispatcher.dispatch(this, registry, event, topic);
}

@Override
public void close() {
this.dispatcher.close();
}

@Override
public String getBusName() {
return this.busName;
}
}

Dispatcher 实现

Dispatcher 用于执行消息消费,因此对于 Dispatcher,它需要直到要消费的消息信息及异常处理器:

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import java.lang.reflect.Method;
import java.util.Objects;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.4 14:33
* @description: 消息执行器
*/
public class MyDispatcher {

private final Executor executorService;

private final MyEventExceptionHandler eventExceptionHandler;

public static final Executor SEQ_EXECUTOR_SERVICE = SeqExecutorService.INSTANCE;

public static final Executor PER_THREAD_EXECUTOR_SERVICE = PerThreadExecutorService.INSTANCE;

private MyDispatcher(Executor executorService, MyEventExceptionHandler eventExceptionHandler) {
this.executorService = executorService;
this.eventExceptionHandler = eventExceptionHandler;
}

public void dispatch(MyBus bus, MyRegistry registry, Object event, String topic) {
ConcurrentLinkedQueue<MySubscriber> subscribers = registry.scanSubscriber(topic);
if(Objects.isNull(subscribers)) {
if(Objects.nonNull(eventExceptionHandler)) {
eventExceptionHandler.handle(new IllegalArgumentException(String.format("The topic [%s] not bind yet", topic)),
new DefaultMyContext(bus.getBusName(), null, event));
}

return;
}

// 遍历所有的监听者,拿到对应的事件参数对象,比对消息的事件类型和监听者参数事件类型是否一致
subscribers.stream().filter(subscriber -> !subscriber.isDisabled())
.filter(subscriber -> {
Method subscribeMethod = subscriber.getSubscribeMethod();
Class<?> parameterTypeClass = subscribeMethod.getParameterTypes()[0];
return parameterTypeClass.isAssignableFrom(event.getClass());
})
.forEach(subscriber -> invokeRealSubscribe(bus, subscriber, event));
}

private void invokeRealSubscribe(MyBus bus, MySubscriber subscriber, Object event) {

Method subscribeMethod = subscriber.getSubscribeMethod();
Object subscribeObject = subscriber.getSubscribeObject();

this.executorService.execute(() -> {
try {
subscribeMethod.invoke(subscribeObject, event);
} catch (Exception e) {
if(Objects.nonNull(this.eventExceptionHandler)) {
this.eventExceptionHandler.handle(e, new DefaultMyContext(bus.getBusName(), subscriber, event));
}
}
});
}


public static MyDispatcher newDispatcher(Executor executorService, MyEventExceptionHandler eventExceptionHandler) {
return new MyDispatcher(executorService, eventExceptionHandler);
}

public static MyDispatcher newSeqDispatcher(MyEventExceptionHandler eventExceptionHandler) {
return new MyDispatcher(SEQ_EXECUTOR_SERVICE, eventExceptionHandler);
}

public static MyDispatcher newPerThreadDispatcher(MyEventExceptionHandler eventExceptionHandler) {
return new MyDispatcher(PER_THREAD_EXECUTOR_SERVICE, eventExceptionHandler);
}

public void close() {
if(executorService instanceof ExecutorService) {
((ExecutorService) executorService).shutdown();
}
}

private static class SeqExecutorService implements Executor {

private static final SeqExecutorService INSTANCE = new SeqExecutorService();

@Override
public void execute(Runnable command) {
command.run();
}
}

private static class PerThreadExecutorService implements Executor {

private static final PerThreadExecutorService INSTANCE = new PerThreadExecutorService();

@Override
public void execute(Runnable command) {
new Thread(command).start();
}
}

public static class DefaultMyContext implements MyEventContext {

private final String busName;

private final MySubscriber subscriber;

private final Object event;

public DefaultMyContext(String busName, MySubscriber subscriber, Object event) {
this.busName = busName;
this.subscriber = subscriber;
this.event = event;
}

@Override
public String getSource() {
return this.busName;
}

@Override
public Object getSubscriberObject() {
return Objects.nonNull(subscriber) ? subscriber.getSubscribeObject() : null;
}

@Override
public Method getSubscribeMethod() {
return Objects.nonNull(subscriber) ? subscriber.getSubscribeMethod() : null;
}

@Override
public Object getEvent() {
return this.event;
}
}
}

MyRegistry 实现

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import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.4 14:04
* @description: 已订阅bus的订阅者容器对象
*/
public class MyRegistry {

private final ConcurrentHashMap<String, ConcurrentLinkedQueue<MySubscriber>> subscriberContainer = new ConcurrentHashMap<> ();

/**
* 绑定
* @param subscriber
*/
public void bind(Object subscriber) {
List<Method> subscriberMethods = getSubscriberMethods(subscriber);
subscriberMethods.forEach(method -> tireSubscriber(subscriber, method));
}

private void tireSubscriber(Object subscriber, Method method) {
MySubscribe mySubscribe = method.getDeclaredAnnotation(MySubscribe.class);
String topic = mySubscribe.topic();
subscriberContainer.computeIfAbsent(topic, key -> new ConcurrentLinkedQueue<>());
subscriberContainer.get(topic).add(new MySubscriber(subscriber, method));
}

private List<Method> getSubscriberMethods(Object subscriber) {
List<Method> methods = new ArrayList<>();

Class<?> subscriberClass = subscriber.getClass();
while (Objects.nonNull(subscriberClass)) {
Method[] declaredMethods = subscriberClass.getDeclaredMethods();

Arrays.stream(declaredMethods).filter(method ->
method.isAnnotationPresent(MySubscribe.class) &&
method.getParameterCount() == 1 &&
method.getModifiers() == Modifier.PUBLIC
).forEach(methods::add);

// 获取当前 subscriber 的父类
subscriberClass = subscriberClass.getSuperclass();
}

return methods;
}

/**
* 解绑
* @param subscriber
*/
public void unBind(Object subscriber) {
subscriberContainer.forEach((key, queue) -> {
queue.forEach(subscriberItem -> {
if(subscriberItem.getSubscribeObject() == subscriber) {
subscriberItem.setDisabled(false);
}
});
});
}

/**
* 根据topic获取已经绑定的监听对象
* @param topic
* @return
*/
public ConcurrentLinkedQueue<MySubscriber> scanSubscriber(final String topic) {
return subscriberContainer.get(topic);
}

}

EventExceptionHandler 异常处理器

当有异常发生时,用户可以自己实现异常处理器接口,当有异常发生时会回调该接口:

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/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.4 14:14
* @description: 异常处理器
*/
public interface MyEventExceptionHandler {

/**
* 对 event 异常处理
* @param cause
* @param eventContext
*/
void handle(Throwable cause, MyEventContext eventContext);

}

消息上下文对象

异常产生,需要告知消费者,产生异常的相关上下文:

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import java.lang.reflect.Method;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.4 14:28
* @description:
*/
public interface MyEventContext {

String getSource();

Object getSubscriberObject();

Method getSubscribeMethod();

Object getEvent();

}

Subscribe 监听器注解

监听器注解用于标记要监听并执行的方法:

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/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.4 15:00
* @description: 自定义监听器订阅方法注解
*/
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MySubscribe {

String topic() default "default-topic";
}

监听者信息对象

为了便于管理监听者对象,需要对监听者的信息进行封装,并存储到 MyRegistry 的 ConcurrentHashMap<> 容器中:

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import java.lang.reflect.Method;

/**
* @projectName: guava
* @author: woodwhales
* @date: 20.7.4 15:05
* @description: 监听者信息对象
*/
public class MySubscriber {

private final Object subscribeObject;

private final Method subscribeMethod;

private boolean disabled = false;

public MySubscriber(Object subscribeObject, Method subscribeMethod) {
this.subscribeObject = subscribeObject;
this.subscribeMethod = subscribeMethod;
}

public Object getSubscribeObject() {
return subscribeObject;
}

public Method getSubscribeMethod() {
return subscribeMethod;
}

public boolean isDisabled() {
return disabled;
}

public void setDisabled(boolean disabled) {
this.disabled = disabled;
}
}

小结

自实现一个简单的 EventBus,要涉及到以下核心组件:

核心接口

MyBus

通过构造器接收监听者(使用 @MySubscribe 注解了某些方法)和自定义异常处理器(自实现了 MyEventExceptionHandler 接口)

MyEventExceptionHandler

用于消息过程出现异常时,处理异常

MyEventContext

异常处理过程中,将产生异常的数据封装起来

核心类

MyEventBus

MyBus 接口的实现类,其内部维护了 MyDispatcher 和 MyRegistry。

MyDispatcher

用于真正指定消费逻辑,内部维护了 MyEventExceptionHandler 实现(从 MyEventBus 构造器传入的)。

MyRegistry

用于对所有加了 @MySubscribe 注解的监听者信息进行管理,包括注册和注销。

MySubscriber

对注册的监听者信息进行封装,方便 MyRegistry 存储,也方便 MyDispatcher 消费执行。

核心注解

MySubscribe

提供要监听消息的方法注解标记。

updated updated 2020-07-14 2020-07-14
本文结束感谢阅读

本文标题:Guava EventBus 使用介绍及自实现详解

本文作者:木鲸鱼

微信公号:木鲸鱼 | woodwhales

原始链接:https://woodwhales.cn/2020/07/06/072/

许可协议: 署名-非商业性使用-禁止演绎 4.0 国际 转载请保留原文链接及作者。

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